Texas Instruments TNETX4090 specifications Ieee Std 802.1Q tags ± reception, Address maintenance

TNETX4090

ThunderSWITCH II 9-PORT 100-/1000-MBIT/S ETHERNET SWITCH

SPWS044E ± DECEMBER 1997 ± REVISED AUGUST 1999

IEEE Std 802.1Q tags ± reception

By the time the IALE examines the received frame, it contains an IEEE Std 802.1Q tag header (after the source address). The tag used depends on the port configuration. If the port is configured as an access port, IALE always uses the default VID programmed for this port and assumes that all received frames on this port are untagged. If the frame already contained a tag, it is tagged again. If the port is programmed as a non-access port, the tag added depends on the received frame. If the frame is not tagged or the value of the tag field is 0x000, the default port VID is used to internally tag the frame. Otherwise, the VID contained in the frame is used by the IALE.

The IALE supports 64 of the possible 4096 VIDs that can be encoded within the IEEE Std 802.1Q tag. The VID in the received frame is compared with these 64 VLAN IDs to see which (if any) of them it matches.

unknown VLAN

If there is no match, the rest of the address-lookup process is abandoned. A new VLAN interrupt is provided to the attached CPU. The source address, VLAN ID, and port information is provided in internal registers so that the CPU can determine if it wants to add this VID to the lookup table. If the destination address is unicast, the frame is discarded. If the destination address is a multicast/broadcast, the frame is forwarded based on a programmable port mask.

known VLAN

If there is a match, the VLAN index associated with this VID, together with the destination and source address, are forwarded to the address lookup and subsequent routing process. (Only one of the VIDs matches if they have been programmed correctly. If more than one matches, the hardware chooses one of them.)

new VLAN member

The IALE checks to see if the source port already has been declared as a member of this VLAN. If not, an interrupt is provided to allow the attached CPU to add this port as a new member of the VLAN.

IEEE Std 802.1Q header ± transmission

The IEEE Std 802.1Q header is carried within the frame to the transmitting MAC port, where the decision to strip out the header before transmission is made, based on the port configuration. If the port is configured as an access port, the tag is stripped before transmission. If the frame is only 64 bytes long, four bytes of pad (0s) are inserted between the end of the data and the start of the CRC word (a new CRC value is calculated and inserted in the frame). Three, two, and one byte(s) are inserted for 65-, 66-, and 67-byte frames, respectively. If the port is configured as a nonaccess port, the VID is compared with the default port VID. If they match, the header is stripped; otherwise, the header is retained.

If the frame is transmitted to the NM port, no header stripping occurs; the frame is transmitted unaltered. It may contain one or two IEEE Std 802.1Q headers, depending on how the frame is received.

address maintenance

The addresses within the IALE can be maintained automatically by the TNETX4090, where addresses are learned/updated from the wire and deleted using one of two aging algorithms looked up during frame-routing determination. Multicast addresses are not automatically learned or aged. The attached CPU can add/update, find, or delete address records (see TNETX4090 Programmer's Reference Guide, literature number SPAU003, for details) via the DIO interface.

The learning and aging processes are completely independent. This allows addresses to be automatically learned from the wire, but allows the CPU to manage the aging process under software control.

spanning-tree support

Each port provides independent controls to block reception or transmission of frames, learning of addresses, or disable the port on a per-port basis. Blocking can be overridden to allow reception or transmission of spanning-tree frames.

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